1. Field of the Invention
This invention relates to a concrete composition for making concrete modlings of high strength and to a method for making such concrete moldings.
2. Prior Art
Hithertofore, concrete moldings such as a concrete pile have been generally made by forming moldings of ordinary Portland cement, elevating the temperature at a rate of 15.degree. C./3 hrs. up to 60.degree.-65.degree. C., then maintaining the moldings within the above temperature range for four hours, curing in water after cooling and subsequently effecting exposure curing. In this method, however, three to four weeks are required before product concrete moldings of predetermined design strength or compressive strength of about 800 kg/cm.sup.2 or higher are obtained. In order to obtain such concrete moldings in a relatively short period of time, autoclave curing has to be further effected approximately at 180.degree. C. under 10 atm. after steam-curing. However, this procedure has a disadvantage in that pre-stress transferred during autoclave curing tends to decrease. Particularly, the larger the initial pre-stress is, the more remarkably this phenomenon is exhibited. In some cases, more than 50% of the initial pre-stress is killed. This means that desired concrete moldings of high strength cannot be obtained merely by increasing compressive strength. It is, therefore, necessary to transfer a proper order of pre-stress in association with the order of compressive strength. Alternatively, as concrete moldings which have been placed at an elevated temperature of 180.degree. C. are quenched after curing, the water content is abruptly evaporated with resultant sudden shrinkage in the concrete moldings. Therefore, it sometimes occurs that the reduction in weight of the concrete moldings to 5-6% by weight so that the moldings are cracked due to temperature difference between the inner portion and the outer surface thereof. For these reasons, according to the conventional method it has been impossible to make concrete moldings of high strength with reduced relaxation in a short period of time. Alternatively, regulated set cements are known from U.S. Pat. Nos. 3,864,138, 3,867,163 and 3,954,489. This type of cement which contains 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 (X represents halogen) and anhydrous calcium sulfate exhibits high strength in a short period of time. However, since this cement contains 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 having highly active reactivity, concrete having proper workability cannot be obtained when the cement is mixed at a low water/cement ratio. At such a water/cement ratio, high strength cannot be obtained, either. Further, even if the concrete is subjected to steam curing, the strength remains unchanged. It is considered this is because when mixed with water, highly active 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 contained in the regulated set cement causes hydration reaction independently fairly faster than cement (calcium silicate) to thereby form calcium aluminate tri-sulfate hydrate and because the thus formed calcium aluminate tri-sulfate hydrate is converted to calcium aluminate mono-sulfate hydrate (3CaO.Al.sub.2 O.sub.3.CaSO.sub.4.12H.sub.2 O) by steam curing so that fine precise construction of the concrete is broken.